Mud logging has been used for a long time in petroleum drilling to determine the approximate location of gas bearing strata during the drilling process. In particular, mud logging involves the process of examining the drill cuttings extracted from the drilling mud to identify gas, hydrocarbon and other constituents which exist at the particular location of the drill bit. To that end, a gas detector is usually set up at the surface to sample the outflow of the drill mud from the borehole. This location is frequently above a shale shaker, but may be installed at other locations. The sampling equipment detects gases released from the drilling mud along with air that is drawn in by the sampling equipment. The system provides a qualitative analysis of the gases being released from the borehole. If the mud logging system monitors the progress of drilling operation and the drilling mud flow rate, it is possible to calculate the approximate location in the borehole where the gas was released. This process involves a calculation of up-hole velocity with time and its correlation with the output of the mud logger.
The conventional mud logging systems lack quantitative estimates of gas release volume because of the nature of the sampling process where air is drawn from above the mud in the belly of a shaker or some other area. In the case of a reservoir such as coal or shale where the gas is contained within the rock itself, the gas volume release can be expected to be directly related to the volume of rock drilled and directly related to the gas content of the coal on a volume per volume basis. This also applies to all the gas bearing sediments without large pore space such as vugs. In the latter case the gas release would be expected to extend beyond the drilled volume of the hole.
The usual method for obtaining gas constituents from coal seams is to core drill into the coal seam and pull the core as quickly as possible to the surface. The core is then removed from the core barrel and placed within a canister where desorption of the gas from the core sample is monitored. Invariably, gas is lost during the transit period from the depth of the coal seam to surface. This lost gas must be calculated from backwards extrapolation of the initial desorption rate of the core once it is placed in the canister, to the time at which it is considered that the coal commenced desorption. As the gas released from the core slows down, it is customary to open the canister and sample the core, then crush the sample to expedite the desorption process. The gas released from the crushed sample is measured and used in the analysis of the total gas content of the core sample. This measurement is usually specified as a gas volume per unit weight of coal.
The limitations of this technique involve the requirement to conduct a coring process to obtain a core sample, as well as the inaccuracies in the estimation of the initial gas lost to the atmosphere during the analysis procedure. It can be seen that a need exists for a process in which the analysis of the gas constituents can be obtained during a conventional drilling of the strata, where the drill mud with the cuttings therein is not exposed to the atmosphere, but is contained until the gas analysis is completed. Yet another need exists for a gas analysing system that is dynamic, meaning that the gas is continually accumulated and analysed.